1. Field of the Invention
The present invention relates to a coil spring which can be used in a suspension of a vehicle, for example.
2. Description of the Related Art
As a method of producing a coil spring, a method of forming the coil spring by hot working and a method of forming the coil spring by cold working are known. The coil spring formed by hot working is formed into a helical shape by winding a wire that is heated to a high temperature (for example, an austenitizing temperature of steel) around a mandrel at a predetermined pitch. The length of the wire is equivalent to that of one coil spring. In this specification, an apparatus which produces a coil spring by hot working is referred to as a hot forming coiling machine, and the coil spring formed by hot working is referred to as a hot formed coil spring. Since the hot forming coiling machine winds a wire which has been softened as a result of heating around the mandrel, it is suitable for producing a coil spring having a relatively large wire diameter.
In contrast, a coil spring formed by cold working is produced in such a way that a wire having a length equivalent to that of multiple coil springs is supplied between a first pin and a second pin of the coiling machine, and the wire is formed to be arc-shaped continuously between the first pin and the second pin. When the wire is formed into one coil spring, the wire is cut by a cutter. In this specification, an apparatus which produces a coil spring by cold working is referred to as a cold forming coiling machine, and the coil spring formed by cold working is referred to as a cold formed coil spring. The cold forming coiling machine can produce a coil spring of a special shape other than a cylindrical shape.
FIG. 13A schematically shows an example of a conventional coil spring 100. A lower end turn portion 100a of the coil spring 100 is supported by a lower spring seat 101. An upper end turn portion 100b of the coil spring 100 is supported by an upper spring seat 102. The positions of the spring seats 101 and 102 are determined in accordance with the specifications of the counterpart components which support the coil spring. Since the wire is wound helically, the coil spring 100 is not symmetric with respect to the center of the coil. Accordingly, when a compressive load F1 is applied to the coil spring 100, it is known that an effective portion 100c is deformed in a direction shown by arrow f1, and that the so-called bowing occurs. The bowing tends to occur particularly in a coil spring having a small number of turns (for example, when the number of turns is five or less). The coil spring 100 having the bowing may interfere with the surrounding components, and thus, such coil spring 100 is not desirable.
In order to prevent the occurrence of bowing, as indicated by another conventional example shown in FIG. 13B, a coil spring 110, which is formed in a C-shape or an S-shape in its free shape not subjected to application of load, has also been proposed. The aim is to make an effective portion 110c straight as the effective portion 110c of the coil spring 110 is deformed in a direction shown by arrow f2 in a state in which the compressive load F1 is applied. JP 1989-156119A or JP 2642163B also discloses a coil spring formed in a C-shape. Since it is sometimes difficult to actually produce a coil spring of such a special shape by a hot forming coiling machine, the cold forming coiling machine is used. However, when a coil spring having a large wire diameter is to be produced by the cold forming coiling machine, particular measures regarding the structure and control of the coiling machine must be taken.